1. Technical Field
This invention relates to the art of making bipartite connecting rods by cracking, and more particularly doing so with a forged wrought steel connecting rod possessing typical ductility.
2. Discussion of the Prior Art
Connecting rods have been made bipartite for some time by sawing the enlarged end of the connecting rod into two pieces and reassembling the sawed pieces with bolts (see U.S. Pat. Nos. 1,831,325 and 2,371,400). Fracturing is a more attractive alternative because it allows the irregular jagged ridges on the cracked plane to facilitate more exact rematching of the pieces. One of the earliest patented approaches to cracking forged wrought steel rods is disclosed in U.S. Pat. No. 2,553,935 (to Parks et al, 1951). The steel used for connecting rod in the 1940's was tough and ductile. To encourage cracking of such wrought steel, the cross-sectional area of the cracking plane was reduced by Parks with saw kerfs and drilled holes. This avoided using heat treatment to make the steel more brittle, which heat treatment would discourage ease of subsequent finish machining and would degrade the toughness of the connecting rod. This also avoided use of other brittlyzing techniques, such as cryogenics, which is exorbitantly expensive. An extension of the Parks idea of reducing the cross-sectional area is also shown in the later U.S. Pat. No. 4,693,139 , employing dual V-notches to make the initiation of cracks easier.
The Parks patent, as well as U.S. Pat. Nos. 4,569,109; 4,768,694; and 4,860,419, have used a mode of cracking that employs a mandrel placed in the big bore of the connecting rod; the mandrel is expanded by forcing a wedge sideways therethrough. Wedge expandable mandrel cracking is disadvantageous because it is difficult to control cracking uniformity and lacks repeatability of quality when processing large volumes of connecting rods. Slight material variations or dimensional discrepancies between rods will require a slightly different wedge or stroke of the wedge to achieve uniformity. Such control of the wedge cannot be easily obtained.
Application of a cracking force has been tried in other modes, such as in U.S. Pat. No. 3,994,054, wherein conical members were thrust into smaller but separate openings drilled transversely through the cracking plane of the connecting rod. This mode resulted in uncontrolled cracking since the greatest force would occur unevenly on the side of the connecting rod where the conical members first entered the openings.
What is needed is a reliable method of consistently making excellently balanced and smoothly cracked connecting rods with the method operating on a one-piece forged wrought steel connecting rod possessing typical ductility and which method eliminates the need for any brittlyzing techniques.